The present invention relates generally to electromigration sensors and more specifically to an improved electromigration velocity sensor and electromigration failure warning device.
The effect that scratch marks on metal lines underwent current induced migration was incorporated into an edge displacement method by Blech and Kinsbron as discussed in "Thin Solid Films," Vol. 25, pages 327, 1975. This method was refined to produce the device of FIG. 1 which was reported by English and Kinsbron in the Journal of Applied Physics, Vol. 54, page 268 in 1983. The electromigration detector of FIG. 1 includes a substrate for example, a silicon, covered by an insulative layer of silicon dioxide and silicon nitride. On the top silicon nitride layer is a first conductive layer having a low electron migration rate. This layer may be for example, a titanium-nitride alloy. Placed on the titanium-nitride alloy is a cathode and an anode electrode spaced from each other and of a material having a higher electromigration rate. Such materials for the cathode and anode may be for example, aluminum. A potential is applied to the cathode and anode and electrons travel from the cathode through the tungsten-nitride alloy to the anode. Due to the high conductivity and electromigration rate of aluminum, the electrons are injected from the transition portion of the tungsten-nitride alloy into the aluminum. This anode edge then migrates to the right in FIG. 1 using the aluminum as a material source. The edge displacement of the aluminum anode is interpreted as the average aluminum atom velocity. Its migration velocity is determined electrically by measuring the change of resistance between the cathode and the anode. This change of resistance results from the increased length of the tungsten nitride path since the intermediate section migrates to the right as the anode recedes to the right.
Although migration of the anode is prefered because of the ordered manner in which the atomic migration occurs, some movement of the cathode also occurs. The movement of the cathode undesirably effects the measurement ability of the migration velocity.
Electromigration and its relationship to conductor lifetime is discussed in U.S. Pat. No. 3,474,530.
Thus, it is an object of the present invention to provide an improved electromigration velocity sensor which overcomes the problem of the cathode migration.
Another object of the present invention is to provide an electromigration failure warning device.
These and other objects of the invention are attained by providing an improved cathode structure which reduces cathode migration by reducing current density. This is achieved by providing the cathode structure to have a substantially larger transverse width adjacent to the transition or intermediate portion of the first conductive layer compared to the transverse width of the anode structure adjacent to the intermediate portion of the first conductor. The improved electromigration velocity sensor can also be used as an electromigration failure warning device by providing an additional conductor which contacts the anode portion of the first conductor and is separated from the anode by the first conductor. This contact point is substantially adjacent to the intermediate or transition portion of the first conductor. A failure warning is provided by detecting electromigration of the anode conductor past the additional conductor by detecting the rapid change of resistance between the anode conductor and the additional conductor. The additional conductor may include a plurality of conductors which are connected to the anode portion of the first conductor at spaced points along the migration axis of the device. A single contact pad interconnects the additional conductors or fingers and one of the plurality of fingers may be selected with the others being disconnected from the common pad for example, by laser cutting.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.